1. Technical Field
The present invention relates to a discharge lamp for generating light by electric discharge between an internally-provided pair of electrodes, and to a light source device and a projector respectively including the discharge lamp.
2. Related Art
Discharge lamps, each of which includes: an emitting portion in which a pair of electrodes are disposed and discharge space for encapsulating an emitting material such as mercury or rare gas is provided; and a pair of sealing portions respectively extending in a direction away from each other with the emitting portion interposed therebetween, the pair of sealing portions internally provided with electrode-connecting wires connected to the electrodes, have been known. In such a discharge lamp, when a voltage is applied on the electrodes, an electric discharge is generated at a predetermined portion of the electrodes, so that heat generated by the electric discharge is transferred to front ends of the electrodes (i.e., ends close to each other). When a temperature of the front ends of the electrodes reaches a predetermined temperature region, an arc discharge is started between the front ends, thereby reacting the emitting material between the electrodes to emit light.
According to a known arrangement of such an electrode, an electrode includes a shaft (core stick) and a coil wound around the shaft. Since an electric field is concentrated on step portions of the coil of the electrode, an electric discharge tends to be generated at the step portions of the coil. While the electrode can be considerably worn and deformed when the temperature of the electrode becomes excessively high, the coil contributes to an increase in heat capacity, thereby keeping the electrode at a suitable temperature. In addition, by winding the coil around the shaft, a diameter of the electrode can be easily adjusted to correspond to wattage of the discharge lamp.
However, when such a step portion is located at a position closer to the sealing portions, an arc generated by the electric discharge at the step portion may collide on an inner wall of the tube made of a material such as quartz glass. In such an instance, the emitting portion may become white-turbid due to damages on the inner wall, or tungsten, i.e., a material for forming the electrode, may adhere to the emitting portion due to defects in a halogen cycle to cause a tanning thereon, by which a lifetime of the discharge lamp may be shortened. In such a discharge lamp, the inner wall of the emitting portion of the discharge lamp is typically formed in a shape of substantially sphere around the center of a line connecting the electrodes. Thus, particularly when a distance between the inner wall of the emitting portion and a starting point of an electric discharge is short, problems as described above become notable.
According to a known arrangement of a discharge lamp, in view of such problems, a starting point of an electric discharge is arranged to be placed adjacent to a front end of the electrode (for instance, see Document 1: JP-A-2006-79986).
According to the arrangement of the discharge lamp disclosed in the Document 1, a winding portion formed by winding a wire of tungsten in a coiled manner and a curved portion formed by heat-treating and fusing the most rear portion of the winding portion are provided in the vicinity of the front end of the electrode. In the curved portion, a step between the coil and the shaft is reduced. With such an arrangement, the starting point of an electric discharge is placed on a space present in the winding portion, thereby preventing an electric discharge from being started at a rear end of the electrode at which a distance between the inner wall of the discharge lamp and the electrode is short.
When a discharge lamp is lighted for a long time, tungsten having been freed from the electrode forms sharp crystals on a portion of the shaft adjacent to the rear end of the electrode, so that an electric discharge may be started from the crystals, thereby hindering the starting point of an electric discharge from being specified.
More specifically, when an arc collides on the inner wall of the discharge lamp, silicon oxide for forming the tube of the discharge lamp is vaporized to be separated into silicon and oxygen, and the separation of silicon oxide causes tungsten forming the electrode to be vaporized. Then, among the vaporized tungsten, tungsten halide having been placed out of a halogen cycle is convectively delivered to the vicinity of the shaft (core stick), by which tungsten atoms are considered to be accumulated on the shaft. As described above, the starting point of an electric discharge in the electrode tends to be placed on step portions that project radially outward from the electrode. Thus, when tungsten is accumulated and crystallized to form a sharp shape, the starting point of an electric discharge when lighting of the lamp is initiated can be placed on the portion on which tungsten is accumulated. Hence, even if the starting point of an electric discharge is attempted to be specified on a portion of the coil by providing a curved portion in the vicinity of the rear end of the coil as in the arrangement of the discharge lamp disclosed in the Document 1, the starting point of an electric discharge may not be specified due to a long-time lighting of the discharge lamp.